The present invention relates to resistance welding controllers and in particular to a resistance welding controller that is adapted to monitor various parameters associated with the weld and dynamically control the welding current and weld time accordingly to achieve the desired weld.
With the increasingly widespread use of automated mechanisms, more commonly referred to as "robots", to perform welding operations, the need has arisen for more sophisticated welding controllers which are capable of consistently performing uniform high quality welds while operating unattended over extended periods of time, despite changes in such factors as line voltage, material thickness and electrode diameter. Moreover, in view of the time constraints imposed on assembly line operations, it is additionally important that automated welding controllers be able to consistently achieve the desired weld quality without significantly altering the target weld time.
It is well known in the resistance welding art that the quality of a weld is related to the change in resistance that occurs between the workpieces during the formation of the weld. This characteristic is referred to hereinafter as the .DELTA.R value of the weld and refers to the percentage change in weld nugget resistance between the maximum resistance value experienced and the resistance value at weld termination. Accordingly, resistance welding controllers have been proposed which monitor the resistance between the weld electrodes and terminate the weld when a preselected .DELTA.R value is sensed. An example of a welding control mechanism of this type is disclosed in U.S. Pat. No. 3,586,815.
The disadvantage with welding controllers of this type, however, is that weld quality is controlled solely by controlling weld time. Consequently, under certain conditions, such as with "mushrooming" electrodes, weld time may need to be significantly extended beyond the target weld time in order to achieve the desired .DELTA.R value. This can present significant problems, particularly when operating under an assembly line production schedule. For example, with a target weld time of 15 cycles, a weld time of 20 cycles may be unacceptable.
Accordingly, it is the primary object of the present invention to provide a resistance welding controller that is adapted to monitor weld nugget resistance and dynamically control both weld current and weld time to achieve a desired .DELTA.R value. In this manner, weld quality is maintained without significantly affecting weld time. Specifically, the present resistance welding controller is adapted to monitor weld nugget resistance values and rates of resistance change during the early stage or "heat-up" phase of the weld, and compare the sensed resistance characteristics to a predetermined target resistance curve. If the results of the comparisons indicate a projected weld time significantly greater than or less than the target weld time, then a dynamic change is made in weld current--i.e., percentage heat. With the correction in weld current made, the controller is then adapted to continue to monitor the weld nugget resistance and terminate the weld when the desired .DELTA.R value has been satisfied. The following weld is then automatically initiated at the revised percentage heat setting and the target resistance curve is updated based upon the experience of the most recent successfully completed welds.
In addition, the resistance welding controller according to the present invention is further adapted to compare upon weld termination the actual weld time experienced with the target weld time and make an additional correction in weld current in preparation for the next weld if the weld was terminated either beyond or short of the target weld time. Thus, it will be appreciated that while weld time is still permitted to vary in order to satisfy the desired .DELTA.R value, the changes in weld time are small due to the dynamic control of weld current.
Furthermore, the preferred embodiment of the present welding controller is also adapted to detect during the heat-up phase of the weld if the electrodes have been freshly dressed and respond by immediately reverting to the original weld schedule. In this manner, the necessity of having to re-initialize the welding controller each time the weld electrodes are serviced is avoided.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :